Stir together flour, sugar, baking powder and salt in a medium mixing bowl. If using frozen blueberries, add them to the flour mixture. Make a well in the center of the mixture and set aside.

Combine eggs, milk, sour cream, butter, and orange peel. Add all at once to the dry mixture. Stir just until moistened (batter should be lumpy). If using fresh blueberries, fold in now.

Spoon batter into prepared muffin cups, filling each almost full. If desired, sprinkle tops with coarse sugar. Bake in a preheated oven 20 minutes, or until golden. Remove from the oven and cool in muffin cups on wire rack 5 minutes. Remove from muffin cups and continue to cool.